Foams which contain urethane groups are widely used, for example, in the field of insulation for the manufacture of structural elements and for upholstery purposes. It is known that cold setting foams which contain urethane groups can be produced from higher molecular weight polyols such as hydroxyl polyethers, special polyisocyanates, water and/or other blowing agents, optionally in the presence of catalysts and other auxiliary agents. The higher molecular weight polyols used are frequently polyethers, for example, having molecular weights of 400-10,000, which contain at least two hydroxyl groups and in which at least about 10% of the hydroxyl groups are primary hydroxyl groups. These polyethers are used, for example, in combination with special polyisocyanates. The special isocyanates used may be, for example, a mixture of diphenyl methane diisocyanates and polyphenyl-polymethylene polyisocyanates.
The previously known cold setting foams based on polyphenyl-polymethylene polyisocyanates, for example, those described in German Offenlegungsschrift No. 2,425,657, (see particularly Examples 1-5) have a serious disadvantage, which is particularly disturbing when the foams are produced inside molds. Thus, even when the reaction mixture has been inside the mold for about 10 minutes, the foams formed during this time are still irreversibly deformable. Although this tendency to deformation can be overcome by storage of the reaction mixture for about 24 hours or by subsequent heating of the foams after their removal from the mold, for about thirty minutes at 120.degree. C., it is always a disadvantageous characteristic. Furthermore, molding times of less than five minutes cannot generally be achieved.
An improved process for the production of cold setting, flexible foams which contain urethane groups has now been found. This process reduces or substantially obviates the disadvantages described above and is based on the use of a certain mixture of diphenyl methane diisocyanates and oligomer polyphenyl-polymethylene polyisocyanates.